It is well known in the art to recover metals from solutions by an electrolytic process. This process, simply stated, involves immersing a pair of electrodes in an electrolytic solution containing the metal to be recovered and impressing across the electrodes a voltage of sufficient magnitude to cause electrolytic deposition of the desired product.
This process has long been used in the recovery of silver from used photographic processing solutions. In the processing of exposed photographic films or papers, various silver salts employed in their manufacture are dissolved in the aqueous fixative or stop solutions as complex silver salts. If the silver content of these processing solutions is allowed to rise above a certain value, their chemical action becomes unsatisfactory. The solutions must then be replaced. By removing silver from the solutions while the processing is carried on, the life of the solutions can be greatly extended, thereby reducing the cost of the process. An added benefit is the revenue obtained from the sale of the recovered silver.
Prior art electrolytic methods for recovering silver from film processing solutions have suffered from a number of drawbacks. One is the contamination of the processing solution. Another is the release of obnoxious gases. As the concentration of silver in solution is reduced, the electroplating current is carried increasingly by sulphate ions, resulting in the disintegration of these ions into sulfide ions and other complex ions of varying nature. This decomposition contaminates the processing solution, as the sulfide ions attack the plated silver to produce a silver sulfide. The resulting impairment of the purity of the silver is accompanied by the release of hydrogen sulfide gas.
Attempts to control the plating current to minimize these drawbacks have met with only limited success. For example, U.S. Pat. No. 3,875,032 to Thompson shows a system for measuring the concentration of silver in solution using dedicated measuring electrodes which are excited with a fixed voltage. The resultant current is used to control the current applied to the primary plating electrodes. Similarly, U.S. Pat. No. 3,616,412 to Gnage shows a system for measuring silver concentration by determining the time required for the resistance between two cathodes in the solution to drop below a predetermined value. This measurement is then used to control plating current flow. Both of these systems, however, still result in some contamination of the solution and in the release of hydrogen sulfide gas due to their occasional use of high plating voltages.
U.S. Pat. No. 4,612,102 to Brimo discloses an electroplating power supply employing two current set points, one high (e.g. 0.85 amperes) and one low (e.g. 0.2 amperes). When the current delivered to the electrodes exceeds the high set point, the circuit considers there to be enough silver in solution for electroplating to occur. As plating continues and silver is removed from solution, the amperage drops. When it drops below 0.85 amperes, the apparatus causes the applied voltage to drop, dropping the associated current to a level below the low set point (e.g. 0.1 amperes). However, the circuit continues to monitor current and when the current at this low voltage rises above the low set point (0.2 amperes), the circuit kicks back into high voltage (and current), i.e., into the plating mode. In this mode, the plating process continues until the amperage drops again below the high set point and the control recycles as before.
The Brimo system suffers from a number of problems. One is the sensitivity, and thus the inaccuracy, of the set points, thereby making system control somewhat inaccurate and unreliable. Another is that there can be quite a buildup of metal in solution from the time the circuit drops into its low voltage mode until it kicks back into its high voltage plating mode. In other words, no plating occurs from the time the amperage drops below 0.85 amperes at high voltage and the time, at low voltage, the amperage rises above 0.2 amperes. During this interval, metal can build up in solution. Since most photographic processes continuously add and remove solution, valuable silver can be lost if the solution removed from the system contains more than a very nominal amount of silver.
Another problem characteristic of prior art systems, which employ a standby state such as Brimo's, arises in the processing of color film fixing solutions. Color film processing uses a bleach which becomes part of the fixing solution. In systems employing a standby state, the bleach removes silver from the cathode during the standby period and thereby reduces the silver recovery.
The prior art also includes various techniques for timing the electroplating process to correspond to the processing of films by a processing apparatus. For example, U.S. Pat. No. 4,280,884 to Babb, et al. normally applies an "idling" current of 0.5 amperes to the plating cathode, which is stepped up to 3 amperes when the film processor is activated by the introduction of film. The current returns to its 0.5 ampere idle value when the processor becomes inactive. Similarly, U.S. Pat. No. 4,026,784 to Rivers shows a timer that activates the plating power supply for a predetermined time period when a new roll of film is introduced into an associated film processor. These techniques, however, suffer in that the electroplating process is not controlled directly by the silver concentration in solution, but rather is controlled indirectly by the introduction of film into the processor. The amount of plating current and the length of time it must be applied to completely plate the metal from solution without contaminating the solution or producing hydrogen sulfide gas can only be estimated in advance. These systems employ no feedback to respond to actual plating conditions.
From the foregoing it will be recognized that the prior art electroplating techniques suffer from a variety of drawbacks.
Accordingly, it is a principal object of the present invention to provide a method and apparatus for efficiently electroplating metal from solution that overcomes drawbacks found in the prior art.
It is another object of the present invention to sense the presence of a predetermined concentration of metal salts in a solution and thereupon switch a plating process to an active state in which metal is plated from the solution.
It is a further object of the present invention to sense depletion of metal salts in a solution and thereupon switch a plating process to a standby state in which metal is neither plated from nor leached back into solution.
It is yet another object of the present invention to provide a variable interval at which the concentration of metallic salts in solution is detected when a plating process is in a standby state.
It is still another object of the present invention to provide a simple regulated plating power supply adjustable over at least a portion of the range zero to two volts.
It is yet another object of the present invention to efficiently plate metallic salts out of solution without generating gaseous byproducts or contaminating the solution.
It is still another object of the present invention to temperature compensate a plating power supply to assure uniform operation.
It is yet another object of the present invention to provide a plating system having adjustable output voltages in the plating and standby modes.
It is still another object of the present invention to achieve the foregoing objects with overcurrent protection.
The present invention achieves these objects, and overcomes deficiencies of the prior art, by providing novel techniques for the efficient and automated extraction of metals from solutions. In the preferred embodiment, the "plating" voltage is removed from the electrodes if the current drawn by the solution falls below a threshold value. Thereafter, a lower "standby" voltage is applied to the electrodes. This lower voltage, however, is periodically restored to its higher "plating" value for brief intervals so that the current at the higher voltage can periodically be sampled. If it is found that the current is once again above the threshold value (indicating that new metal has been added to the solution) the electrode voltage is kept at the higher level until the solution is again depleted of metal to the point that the current drops below the threshold. If, during the brief sampling interval, the current is still below the threshold, the electrode voltage is returned to its lower, "standby" value and another current sample is taken at the next interval. The electroplating power supply desirably provides a well regulated output voltage that is variable over a range of low voltages despite its use of conventional regulator circuits which are unable, by themselves, to provide output voltages in this range.
The regulation of the plating voltage is important if the full benefits of the present invention are to be achieved. If the plating voltage varies due to power line fluctuations or other such causes, the current drawn by the solution would vary proportionately. Such variations could cause the device to erroneously switch from its plating mode to standby mode, or vice versa. Regulation of the plating voltage assures that such mode switching is entirely a function of metals concentration and is not influenced by extraneous factors.
The low plating voltage is also an important aspect of the present invention. The rate at which plating occurs is a function of many factors, but is principally dependent on the concentration of metal in solution and on the plating voltage. The plating voltage, however, cannot be set to an arbitrarily high value because high plating voltages (more than a few volts) introduce several serious problems. As mentioned earlier, one problem is the production of undesirable byproducts, such as silver sulfide and hydrogen sulfide. Another is the undesired plating of other metal objects which may be in contact with the solution, such as metal pipes and the like. Accordingly, despite the tradeoff in plating rate, it is generally desirable to plate with low voltages, such as voltages on the order of one volt. As noted, adjustable regulated power supplies in this voltage range have heretofore been impractical due to the unavailability of low voltage integrated circuit regulators.
Even at the low plating voltage employed by the present invention, damage to both the solution and the power supply itself can occur if certain precautions are not taken. For example, as metal is added to a solution, the conductivity of the solution increases. When this occurs during the plating process, the plating amperage increases proportionately to match the conductivity of the solution. It is contemplated that the amperage of each power supply produced according to the present invention will be limited so as to avoid damage to the particular chemistry in the solutions utilized.
In the event that the metal salts cause a short circuit between the negative and positive plating terminals, the plating amperage would theoretically reach an infinite level. This, of course, would destroy tee power supply. The apparatus of the present invention is able to withstand a continuous shorted condition, or high level plating current, for an indefinite period without sustaining damage.
Unlike prior art devices which apply a plating period for a predetermined time period based on the introduction of film into a processing apparatus, the present invention responds directly to an increase in metals content in the solution. The process then continues to plate metal out of solution until the concentration drops below a predetermined value. The present invention does not rely on projections or estimates of how much current must be applied for how long in order to effect complete metal recovery without contaminating the solution nor producing harmful gases. The system's reliability is enhanced by eliminating the need to provide control circuitry linking the electroplating process to the film processing apparatus.
Metals recovery from color film processing solutions is also enhanced with the present invention. Circuitry can be provided to operate a valve or pump to remove silver-depleted solution from the system before the bleach in the solution has a chance to dissolve the silver from the electrode. In prior art systems such as Brimo, such a technique would flush solution having a significiant quantity of silver from the system.
The method and apparatus of the present invention can readily be used to recover copper, gold, aluminum and the like from their salts in solution. For example, copper can be recovered from copper sulfate found in the effluent from the manufacture of printed boards. Such applications are becoming increasingly important as the Environmental Protection Agency is tightening its limits on metallic discharges in industrial waste water.
The foregoing and additional objects, features and advantages of the present invention will be more readily apparent from the following detailed description of a preferred embodiment thereof, which proceeds with reference to the accompanying drawings.